Line a baking tray with oiled parchment – something 20cm square would work well
Add sugar, syrup, margarine and water to a pan
Heat until turns a dark golden brown (think maple syrup) – this should occur at 138 degrees celsius – do this without stirring the mixture, you can shake the pan though
Add vinegar, then bicarb and briefly stir – this will fiercely bubble the mixture
Pour into the lined baking tray and allow to cool – if desired, score before fully cooled to make breaking easier

Ideal as a topping for ice cream if broken into very small pieces, or coat with your favourite chocolate.